This invention relates to heating, ventilation, air conditioning (HVAC) systems and, more particularly, to apparatus for controlling operation of the portion of a system as a function of whether or not an adequate draft is induced in the exhaust so combustion products are discharged from the system.
In conventional HVAC systems, gas is supplied to a furnace portion of the system through a switch controlled valve. The gas is combusted to heat air which is then circulated through the system. The products of combustion, carbon monoxide, etc., are exhausted through a flue or exhaust duct. For this purpose, a draft is induced in the flue or duct by a motor driven fan. The resultant air flow draws off the combustion products from the chamber to the flue. Because the exhaust gases are potentially dangerous, it is important to know if they are, in fact, being exhausted.
There are a number of reasons why they may not be. Among these are a fan motor which is not functioning, a blower fan which is loose, a blocked exhaust flue or duct, etc. Operationally, the HVAC system is controlled so that the gas valve is opened only when a sufficient flow of air is induced in the exhaust. If it is ascertained that combustion gases cannot be properly exhausted, for any of the above reasons, then the valve is not opened.
Various techniques exist for determining if a sufficient air flow is being produced. For example, it is known to monitor the rotational speed of an exhaust motor shaft on which an exhaust fan is installed to insure that the motor is rotating above some predetermined speed. If it is, the gas valve can be opened; otherwise, not. This monitoring device is typically a centrifugal switch which closes when the motor speed exceeds some minimum value, and opens when the speed drops below that level. Such devices have a number of drawbacks. While do they monitor shaft speed, they cannot determine if a fan blade is loose and therefore not able to move a volume of gas. They cannot tell if there is flue blockage. It is possible to employ other devices to monitor for these potential problems. Thus, a pressure sensor installed in the flue could sense a blockage condition. Or, air flow sensors could be used to ascertain flow levels toward the exhaust. However, employing more sensors has an impact in a number of areas. More parts have to be installed, calibrated and adjusted, increasing set-up time. More parts are involved increasing inventory costs. With more parts, the reliability of the system decreases and a certain amount of redundancy may have to be built into the system so that its operating life is maintained. Again, this adds to system cost.
It would be advantageous to have available a single sensor to monitor all these different potential areas of failure; especially, a sensing mechanism whose cost is comparable to that of existing low cost sensors and substantially less than that of other sensors which would otherwise be used.